Hello,
I'm interested in your project, because I have worked for 12 years with PHP and especially with SSO systems.
is it mandatory to work with auth0? I always recommend Apareo CAS for SSO, because is more flexible, we can work today with a database and tomorrow with LDAP, RADIUS, etc.
I have no problem to work with Auth0 too, so feel free to contact me to start as soon as possible.